1. Field of the Invention
This invention relates generally to motors and, more particularly, to detection of turn faults in induction motors.
2. Description of the Related Art
In one method of detecting a turn fault in an AC motor, as described in Kohler et al., U.S. Pat. No. 5,270,640, line current and voltage phasors are determined in an initial step by performing a fast fourier transform (FFT) on current and voltage waveforms. The FFT generally requires several 360 degree cycles. In an additional step, the standard symmetrical component transformation of these phasors is performed to derive the positive and negative sequence voltages and currents from which the impedance phasors can be calculated. Because considerable averaging must be done in finding the phasors and calculating the impedances, the measurement is time consuming and can require at least several cycles. Additionally, the results can be strongly affected by noise.
Conventional implementations of negative sequence impedance methods of on-line turn fault detection have lacked resolution with poor signal-to-noise ratio resulting from considerable noise and harmonics. Furthermore, induced faults often register as increased impedance, which is a phenomena that appears to contradict the notion that the impedance should decrease when shorts occur in a phase. Moreover, when the incoming line has relatively low negative sequence voltages, the impedance becomes indefinite and its value can vary widely.
Most mathematical analyses of turn faulted motors assume a motor that is, except for the fault, symmetric and that is excited by symmetric three phase lines. The complexity and computation required in these models causes them to be unsuitable for evaluating turn faults in unsymmetric motors excited by unsymmetric lines.